1. Field of the Invention
The present invention relates in general to magnetic recording media and more specifically to recording media having highly anisotropic magnetic recording characteristics.
2. Description of the Prior Art
Although performance of reproduction of known recording magnetic media has reached a high level of quality, many uses now exist for magnetic media wherein the usual high quality parameters of such media are less important. For example, in some instances it may be more desirable to provide a magnetic medium that is magnetizable or erasable by only particular magnetic fields than to provide a medium with high reproduction qualities. One highly useful application of a medium magnetizable or erasable by only certain fields is in the area of machine readable magnetically encoded documents such as credit cards.
As society becomes more and more dependent upon the use of magnetically encoded documents as currency substitutes, the need for such documents to be counterfeit resistant and fraud resistant ever increases. Various known magnetically encoded documents are adapted to reduce their use for unauthorized purposes, as evidenced by a U.S. Patent to Holm et al, U.S. Pat. No. 3,566,356, assigned to the assignee of the present invention.
The Holm et al patent discloses a document having a recording medium composed of two magnetizable particle populations that differ from one another in that one population has a relatively low intrinsic coercive force and the other population has a relatively high intrinsic coercive force. The coercive forces of the two particle populations are chosen to provide a gradual demagnetization curve that may be employed to test the validity of the document. Also, Holm et al discloses the use of a sophisticated test for validating the document by the dual recording of information in both populations in a sequence designed to indicate the presence of both high and low coercive force particle populations. Although the Holm et al document provides certain safeguards over nonmagnetic currency substitutes, it is desirable that even more secure currency substitutes are found.